As is well known in the art, microprocessors are large scale integrated circuit devices. A microprocessor chip may contain the control unit, central processing circuitry, and arithmetic and logic circuitry for a computer, permitting its use as a single-chip computer component. As such, microprocessors generally constitute the primary computer element for a computer. Numerous circuit devices are necessary to assist the microprocessor in its function, such as memory chips (RAM and ROM), input-output chips, as well as conventional electrical devices such as diodes, capacitors, inductors and resistors. However, the microprocessor chip will typically be the most expensive single device on a circuit board.
Microprocessor chips are typically soldered to their circuit board, such as a ceramic substrate or printed circuit board. Due to the numerous functions performed by microprocessors, a relatively large number of terminals are required. The size of a typical microprocessor chip is generally on the order of a few millimeters per side, making it somewhat difficult to mount a microprocessor chip to its circuit board. A conventional method adopted by the industry is the flip-chip bonding process. This process utilizes a microprocessor chip referred to as a flip chip, which is generally a monolithic semiconductor device having bead-like terminals provided on one face of the chip. The terminals serve as connections between the microprocessor chip and a suitable conductor pattern formed on the circuit board. Flip chip conductor patterns are typically composed of numerous individual conductors, often as small as about 0.15 millimeter in width, and spaced as little as about 0.2 millimeter apart (i.e., a pitch of about 0.2 millimeter). As a result, soldering a microprocessor flip chip to its conductor pattern requires a significant degree of precision. For this purpose, the terminals are typically formed by using a thick film stencil which selectively prints solder paste on the lower surface of the flip chip in a pattern which is complementary to the conductor pattern on the circuit board. After the stencil is removed, the solder paste is reflowed to form solder bumps by which the flip chip can be reflow soldered to the conductor pattern.
Generally, early in the development of a microprocessor-based product which uses a microprocessor flip chip, the software code required for the fabrication of the production flip chip is not yet established. Consequently, for development purposes the production flip chip's software is typically simulated with a microprocessor emulator, such as a programmable, prepackaged integrated circuit. To do so, a substrate designer is required to design an engineering development unit with an integrated circuit socket mounted to the unit's substrate. A programmable, prepackaged integrated circuit can then be inserted into the socket, by which simulations can be performed for software development. unfortunately, integrated circuit sockets are generally many times larger than a microprocessor flip chip, such that the engineering development unit is required to be much larger than the eventual production unit. Consequently, after the software code has been established, the substrate designer must then specially design a substrate for the production unit, which will utilize the microprocessor flip chip that dictates the production unit's conductor pattern.
Accordingly, a shortcoming of conventional development processes is the requirement that two separate substrates must be designed for a single microprocessor-based product. Such a requirement adds significantly to the time required to arrive at a final production design for the product. Furthermore, the mounting features of an integrated circuit package which incorporates the product, as well as the mounting site within its ultimate working environment, cannot be determined until the size of the final production unit has been established. As a result, the final implementation of the product is further delayed until such considerations have been resolved.
As a solution to the above difficulties, U.S. patent application Ser. No. 08/114861, filed Sep. 2, 1993 and now U.S. Pat. No. 5,438,749, assigned to the assignee of the present invention, teaches a method by which an electronic circuit board that employs a microprocessor flip chip can be developed without the requirement that a development unit be specially designed to accommodate an integrated circuit socket. The method involves the use of a flexible circuit interconnect which can be mated directly with a conductor pattern that is specifically patterned for a circuit board's production flip chip. The production flip chip's conductor pattern is formed on the substrate of an engineering development unit, and the flexible circuit interconnect is registered and soldered to the conductor pattern. The flexible circuit interconnect can then be connected to a microprocessor emulator so as to simulate a microprocessor for the engineering unit, while various development tests and/or evaluations are performed. As a result, there is no requirement to design the engineering unit around an integrated circuit socket, as required in the past, for the purpose of testing and evaluating an engineering unit. Instead, the engineering unit can be designed to accommodate the production flip chip and its conductor pattern, such that a single substrate design can be developed for use in both the engineering development unit and the final production units.
While the method taught by the above-referenced U.S. Ser. No. 08/114,861 can be readily practiced for numerous applications, difficulty is encountered when attempting to solder the flexible circuit interconnect to a conductor pattern whose conductors are spaced less than about 0.35 millimeter apart. The difficulty arises in the use of a thick film stencil for selectively printing solder paste on each of the conductors, thick film or printed, which form the conductor pattern. If the spacing between conductors is less than about 0.35 millimeters, the solder tends to bridge adjacent conductors during reflow in which the solder paste is melted to form the solder bumps with which the flexible circuit interconnect is soldered to the conductor pattern. The resulting electrical shorting renders the screen printing technique unsuitable for the unique test and evaluation method taught by the above-referenced U.S. Ser. No. 08/114,861
Accordingly, what is needed is a method by which the flexible circuit interconnect taught by the above-referenced U.S. Ser. No. 08/114,861 can be more reliably soldered to a conductor pattern on a substrate, particularly when the spacing between conductors is less than about 0.35 millimeters. More specifically, it would be desirable if such a method were able to precisely form a solder bump on each of the conductors of a flip chip conductor pattern, such that the flexible circuit interconnect could be readily reflow soldered to the conductor pattern.